Crustal structures and tectonism in southeastern Alaska and western British Columbia from seismic refraction, seismic reflection, gravity, magnetic, and microearthquake measurements PublicDeposited

Descriptions

Seismic refraction measurements along two unreversed lines
indicate that the earth's crust is 26 km thick in southeastern Alaska
and 30 km thick along the Inside Passage of British Columbia. The
crust in southeastern Alaska, north of Dixon Entrance, consists of
a layer 9 km thick with a seismic velocity of 5.90 km/sec, a layer
7 km thick with a seismic velocity of 6.30 km/sec. and a layer 10 km
thick with a seismic velocity of 6.96 km/sec. The crust along the
Inside Passage of British Columbia, south of Dixon Entrance, consists
of a layer 13 km thick with a seismic velocity of 6.03 km/sec, a layer
5 km thick with a seismic velocity of 6.41 km/sec, and a layer 12 km
thick with a seismic velocity of 6.70 km/sec. The velocity of the
mantle below the M discontinuity is 7.86 km/sec in southeastern
Alaska and 8.11 km/sec in British Columbia.
A compilation of Bouguer gravity data along the Inside Passage
from northern Vancouver Island to northern southeastern Alaska
indicates near-zero anomalies between steep gradients offshore and
near the western margin of the Coast Mountains. A two-dimensional
gravity model, constrained by seismic refraction measurements,
suggests that the thickness of the crust is constant beneath the region
of near-zero gravity anomalies and indicates a step-like transition
between oceanic and continental structure.
Seismic reflection, gravity, and magnetic measurements,
obtained during a 1970 cruise of the R/V Yaquina, help to determine
upper crustal structures in Dixon Entrance. Gravity models, constructed to agree with these data and the measurements of previous
investigators, indicate sediment thicknesses of nearly 3 km east
of Learmonth Bank and west of Celestial Reef. Magnetic models
suggest large lateral changes in basement susceptibility. Either
highly metamorphosed rock or basaltic intrusions can account for
these changes in susceptibility. Folded sediments suggest post depositional
distortion due either to regional compression or to
major local intrusions. Several linear gravity features, observed
in northern Dixon Entrance, disappear north of Graham Island.
Either the structures responsible for the gravity features end or
thick layers of basalt, extending northward from Graham Island,
obscure the effect of the structures.
A single-station survey detected microearthquakes at nine
locations in western British Columbia and southeastern Alaska. The
majority of the observed distant microearthquakes probably originated
in the Queen Charlotte Islands fault zone. However, observed
nearby microearthquakes indicate a microearthquake seismicity of
several events per day along the mainland coast of British Columbia.
Temporary seismic arrays located at a site along the central
portion of Chatham Strait near the Chatham Strait fault and at a site
in Glacier Bay recorded few nearby microearthquakes. Arrivals at
the arrays permitted the location of distant microearthquakes, however,
with epicenters in the vicinity of northern Lynn Canal and along
the Fairweather fault.